Why is it useful to define the return as the sum of the rewards from time $t$ onward rather than up to $t$?
It wouldn't make sense to define the return as you propose, from time 0 to $t$. Once we are in a state at time $t$ we don't care what the returns have been, rather what they will be in the future, thus returns are defined as the total sum of discounted rewards from the current time step onwards. This allows the agent to make decisions about which actions to take based on how valuable taking said action is in the current state at time $t$ -- clearly the rewards previous to this have no effect upon that.